1. Field of the Invention
The present invention relates to a magneto-optic sensor head using the Faraday effect of a bismuth-substituted rare earth iron garnet film.
2. Description of Related Art
A magneto-optic sensor head formed of a bismuth-substituted rare earth iron garnet film is small in size and can be advantageously used in tachometers. Many of industrial apparatuses and consumer products incorporate rotating devices and rotating mechanisms such as motors and gears. Technological advancement have placed strong demands on energy saving and the conservation of global environment and therefore there have been strong needs in the fields of vehicles such as aircraft, ships, and automobiles towards highly accurate, and precise control. In order to control rotating mechanisms and apparatuses with high accuracy and precision, the rotational speeds must be monitored continuously and measured accurately.
Accordingly, a need exists in the art for simple measuring devices which can accurately measure rotational speeds, and such devices should be supplied in large quantity on a commercial base. Various methods of measuring rotational speeds have been proposed, including a method using magnetic induction (SENSOR TECHNOLOGY, December, 1986, p68) and a method using a magneto-optic sensor using the Faraday effect of a magneto-optic material (Applied Optics, Vol. 28, No. 11, 1989, P1992).
The method based on electromagnetic induction is used in measuring the rotational speeds of the engines for aircraft and automobiles. However, this kind of tachometers suffer from a serious drawback that the transmission lines (cables), which run between the sensor and rotating mechanism, are apt to pick up electromagnetic noise.
The probe for detecting a magnetic field is an electrical circuit. The electrical circuit must be of explosion proof construction if the measurement is to be made at a place where dangerous materials such as organic solvents and inflammable materials are manufactured or handled.
In contrast, a magneto-optic sensor formed of a material having the Faraday effect transmits an optical signal and is almost completely free from electromagnetic noise. Thus, the sensor does not have to be of explosion proof construction even if the sensor is used at a premise where organic solvents and inflammable materials are handled.
When light is transmitted through a magneto-optic material placed in a magnetic field, the polarization plane of the light is rotated due to the magnetic field. The sensor converts changes in the rotation of polarization plane into changes in intensity (National Technical Report, Vol. 29, No. 5, p70(1983)).
One practical configuration of a magneto-optic sensor is a transmission type which includes an optical path, a lens, a polarizer, a magneto-optic element, an analyzer, another lens, and another optical path, aligned straight in this order. Another configuration is a reflection type including an optical path, a lens, a polarizer (analyzer), a magneto-optic element, and a mirror (prism), aligned straight in this order. The reflection type lends itself to a miniaturized magneto-optic sensor. The inventors of the present invention have proposed a reflection type magneto-optic sensor that uses a bismuth-substituted rare earth iron garnet with a large Faraday effect as a magneto-optic element (Japanese Patent Application No. 8-184868).
This reflection type magneto-optic sensor incorporates a single optical fiber for inputting and outputting light, a lens, a polarizer, and a bismuth-substituted rare earth iron garnet single crystal film (referred to as BIG) with the film surface placed at an angle with the optical path, and a mirror, all being aligned in this order. When the BIG is positioned with the film surface inclined relative to the optical path, there are a larger number of magnetic domains in an area of the film through which the light passes. The Faraday rotations due to individual magnetic domains cancel out each other when no external magnetic field is applied to the film or the film is not magnetically saturated.
The BIG has the following properties.
(1) When the BIG is not magnetically saturated, it is of a multi-domain structure. The individual magnetic domains are perpendicular to the film surface and adjacent magnetic domains are opposite in polarity.
(2) Faraday rotation angles .theta..sub.F of individual magnetic domains are the same as when the BIG is magnetically saturated, and are opposite in polarity for adjacent domains.
(3) When placed in an external magnetic field, the magnetic domains of the same direction as the external magnetic field become larger in size, and the magnetic domains of the opposite direction become smaller.
(4) When the external magnetic field becomes stronger than a predetermined value, all the magnetic domains are oriented in the same direction.
Light emitting diodes that emit light having wavelengths of 830 nm and 850 nm are inexpensive and readily available. However, it was revealed that when such a light source is used, the reflection type magneto-optic sensor decreases the optical signal strength to a level insufficient for use as a magneto-optic sensor. The inventors conducted research in order to determine what factors cause decreases in signal strength. The inventors found that in order for a sensor to provide a sufficient signal strength despite a large amount (10-14 dB) of absorption of light by the BIG, some special measure must be taken for suppressing the reflection at the connection where the optical fiber is connected to other optical elements. Such special measures are, for example, machining the end surface of the optical fiber in a special manner or application of an anti-reflection film to the end surface of the optical fiber.
FIG. 6 illustrates the light absorption characteristic of a bismuth-substituted rare earth iron garnet single crystal in the near infrared region, and the Faraday effect versus wavelengths (Journal of Applied Physics Vol. 44, p4789(1973)).
Referring to FIG. 6, the Faraday effect of a BIG (Faraday rotation coefficient, .theta..sub.F) is large in short wavelength regions and small in long wavelength regions. FIG. 6 shows that the addition of Bi greatly increases the Faraday effect compared to a rare earth iron garnet single crystal film without Bi.
The optical absorption coefficient is primarily determined by iron ions and not affected by the amount of Bi. The optical absorption coefficient is large in the short wavelength regions and small in the long wavelength regions. Some increase in the optical absorption coefficient is observed at wavelengths around 900 nm. Light having wavelengths longer than 5000 nm is not transmitted through the BIG due to light absorption resulting from lattice vibration.